Various methods have been employed to produce isocyanate compounds. It is known that diamine or monoamine derivatives can be reacted with phosgene at a temperature of about 100.degree. C. for about 28 hours. The by-product hydrogen chloride and excess phosgene are removed by blowing nitrogen through the liquid reaction mixture and recovering the desired mono- and diisocyanates.
In another approach, Klein and Gerhard, Forsch Technol. 1983 CA 100:68763q, report that the unsaturated compound limonene can be treated with hydrogen cyanide and subsequently with phosgene to form a diisocyanate derivative, which is thermally rearranged to diamine dipentene, which in turn, can be treated with phosgene to prepare a diisocyanate of the formula ##STR1## Drawbacks of this process are the toxicity of the hydrogen cyanide and phosgene, and corrosion problems associated with the by-product hydrochloric acid.
The difficulties with direct phosgenations have led to the development of non-phosgenation routes, and these generally involve the pyrolytic thermolysis or cracking of carbamic acid esters or urethanes.
It is also known that isocyanate compounds can be prepared by the reaction of corresponding olefins with isocyanic acid, by reaction of the corresponding halide with an alkali metal isocyanate, and by reaction of the corresponding halide with isocyanaic acid.
Merely by way of illustration, it has been shown in Bortnick, U.S. Pat. No. 2,692,275, that 1,8-diisocyanato-p-menthane can be prepared by pyrolyzing the corresponding carbamate in the presence of a basic catalyst such as the water-soluble metal hydroxides or alkoxides or the water-insoluble metallic oxides or hydroxides.
Mueller and Merten, Chem. Ber. 98, 1097-1110 (1965), carry out the alkylation of urethane, i.e., carbamic acid ethyl ester with a number of cyclic and noncyclic olefins in the presence of acid catalyst to form N-substituted urethanes. The N-substituted urethanes are converted to corresponding isocyanates by "transurethanation", that is, by reacting industrially available higher-boiling mono- or polyisocyanates, e.g., tolylene diisocyanates with the N-substituted urethanes at 200.degree.14 240.degree. C. to release isocyanates.
These various processes are disadvantageous for one or more reasons, such as that the materials are difficult to handle or are corrosive, the yields are poor, expensive reactants are required and the products are difficult to recover.
In Singh, Chang and Forgione, U.S. Pat. No. 4,439,616, tertiary aralkyl isocyanates are produced by thermal cracking of corresponding urethanes formed by the addition of corresponding olefins, e.g., diisopropenyl benzene and carbamic acid esters, e.g., methyl carbamate, at moderate temperatures and in the presence of an acid catalyst. There is no hint or suggestion in Singh et al. that wholly non-aromatic mono- and diisocyanate compounds can be produced by the addition of an olefinic substituted cycloalkene and a carbamic acid ester.
It has now been discovered that wholly non-aromatic cyclohexyl isocyanate compounds can be prepared by a new route of synthesis involving catalyzed pyrolysis of urethanes that have themselves been synthesized by the addition of alkyl carbamate, such as methyl carbamate, to the compound limonene or dipentene [1 methyl-4-(1-methylethenyl)cyclohexene].
It is an important object of this invention to produce cyclohexyl isocyanates utilizing non-corrosive, low-cost starting materials such as limonene or dipentene and methyl carbamate in a simple process yielding the desired isocyanates whereby they are readily recovered and purified.